module sa 
    implicit none
    save        ! 变量是全局的
    integer :: a
end module


program save_
    use sa
    implicit none 
    a = 2
    write(*,*) a
    call change
    write(*,*) a
    call change
    write(*,*) a
        
end program save_

subroutine change
    use sa
    implicit none 
    a = a + a
end subroutine change
